Dryness of the skin is generally caused by a decrease in water content resulting from transepidemic water loss (hereinafter TEWL) which increases due to deterioration of the barrier function of the horny layers of the skin. This is caused by quantitative reduction of lipid secretion, particularly intercellular lipids and natural moisture factors such as amino acids. Therefore, dryness tends to increase in winter, with excessive skin washing, and the reduction of intercellular lipids and amino acids caused by age and state of health. In particular, when the water content in the horny layers has fallen to less than about 10%, the condition is called "dry skin". When skin is in the dried condition, skin gloss tend to decrease, and the appearance of fine wrinkles is observed.
Similarly, in a case of hair, harmful effects such as loss of hair smoothness and deterioration of luster tend to occur with reduction of hair water contents.
In the past, it was desired to prevent the lowering of the water content in the horny layers or hair and to maintain their normal functions. Therefore, various methods have been investigated to achieve such objectives. As a result, one method for inhibiting TEWL is by using occluding agents such as petroleum jelly ointment or emulsions in oil, which give good adhesion to skin and are hydrophobic. Another method for enhancing the hydration effect of cosmetic compositions is by combining a hygroscopic agent and/or a moisture preserving agent. Typically, these agents are selected from polysaccharides, such as hyaluronic acid and chitin; proteins, such as collagen and elastin; polyhydric alcohols, such as sorbitol, ethylene glycol, and glycerol; and organic acid salts, such as sodium pyrolidone carbonate and sodium lactate with a skin or hair treatment base. In addition, it has been found that ceramides and sphingolipids, which are components of the intercellular lipids in the horny layers and the like, play important roles in the preservation of water content, so that the combination of synthetic and/or naturally-extracted ceramides has been practically recommended.
However, the compositions according to the previously-known technique described above are not sufficiently effective in preserving moisture, and further, the use of the occluding agent gives an unpleasant oily or stick feel to the composition, while the hygroscopic agent and moisture preserving agent has a problem of must be used in high concentrations to assure sufficient efficacy in combination, thereby resulting in unpleasant oily and slimy feel. This makes it difficult to establish a suitable formulation which will provide the necessary high dose.
Furthermore, the cosmetic compositions provided according to the previously-known methods are less stable to periodic changes in absorbent conditions and are susceptible to attach by microorganisms.
On the other hand, a substance which can simultaneously provide (1) the hygroscopic and moisture preserving effects of hyaluronic acid and (2) the promotion and stabilization of the formation of the lamellar layer structure (comprising intercellular lipids) of ceramides, which can assist in the hygroscopic and moisture preserving function of the horny layers, has not yet been discovered.
It is an object of the present invention to provide a cosmetic composition which has a beautifying effect on skin and hair and is capable of reversing the damage due to drying such as rough skin, loss of hair luster, and also to provide moisture by preserving sufficient water content in the skin and hair as well as to have a pleasant feel when being applied.
The present inventors have found that the use of a copolymer of 2-methacryloyloxyethylphosphorylcholine and a hydrophobic monomer as components of a cosmetic composition can simultaneously preserve moisture on the skin and protect the hair; they have previously filed a patent application thereon (see Japanese OPI. Hei 5-70321). However, there is a need for a cosmetic composition which will provide still higher water retaining capacity.